Most crops receive the bulk of their annual P and K fertilization at the beginning of the season. Both P and K are notorious for rapidly reduced availability in the following months, but the decline begins within hours to days after application. These decreases in availability are opposite of what crops need as they progress through their development cycle.
Phosphorus and potassium are essential elements needed through the entire season. As crops transition from vegetative growth to reproductive growth, root growth slows and new regions of the root zone are not explored for nutrients like P and K. Nutrient supply becomes even more dependent upon diffusion of nutrients to the roots. Crop vigor may slow without nutrient supplementation.